Ultra-high performance optical systems are required in order to increase the density of integration of the electronic components on a semi-conductor plate and insofar as exposed light of low wavelength (lower than 248 nm) is necessary in order to improve the resolution. The most common technique up to now for obtaining such optical systems uses molten silica. According to another technique, which is already exploited, especially by the companies Bicron and Schott, monocrystals of calcium fluoride and monocrystals of barium fluoride are used. Ultra-high performance far-ultraviolet optical systems with below 200 nm wavelengths require fluoride optical crystals.
Said monocrystals, of calcium fluoride or of barium fluoride, and more generally of alkali metal and/or alkaline-earth metal fluorides, are in principle obtained according to the process known as the Stockbarger-Bridgman process, which is familiar to the person skilled in the art. According to said process, the crystal is generated from an appropriate molten starting material in slowly lowering (generally at a speed between 0.3 and 5 mm/h, more generally between 1 and 3 mm/h) a crucible (or a stack of crucibles) containing said molten material through a solidification zone which is provided in an oven. The crucible(s) is (are) made from a material which is resistant to chemical attack from the material that it contains. In general, it is (a) crucible(s) in graphite of high purity.
According to the teaching of US patents U.S. Pat. Nos. 5,911,824 and 6,093,245, the graphite does have the drawback of being porous (of being a material having open porosity), and it is recommended to coat the internal walls of such graphite crucibles with an appropriate internal coating, in order to <<block the porosity>> of said walls. Carbon coatings, especially pyrolytic or vitreous carbon coatings, are described.
The (mono)crystals must imperatively be prepared in the absence of water, of air and of any other source of oxygen. They are thus generally prepared under vacuum in the presence of a fluorinating agent. Said fluorinating agent ensures the elimination of oxygen, especially of that introduced in the form of oxide as impurity in the starting material. PbF2 is the most utilised fluorinating agent, insofar as its manipulation does not present any particular difficulty, insofar as it is solid at ambient temperature and insofar as it has, itself and its corresponding oxide (PbO), a high vapour pressure at the temperatures of use of crystallisation ovens. Said PbF2 acts, within the context of the preparation of CaF2 crystals, notably according to the reaction:CaO+PbF2→CaF2+PbO.
In practice, it is always delicate to optimise the intervention of said fluorinating agent. It is especially critical: to adjust the rise in temperature of the mixture (for its melting) with the view to said optimisation; to adjust the amount of said fluorinating agent, with the view to minimising any retention of Pb or other fluorinating agent in question in the crystal prepared. Such a retention has obviously disadvantageous repercussions on the performances of transmission and resistance to radiation of said crystal.
In order to obtain high VUV transmission and good laser durability, metal fluoride crystals, for example, CaF2 crystals, must be grown from a high purity starting material. A pretreatment of raw materials is therefore necessary to meet this purity requirement. An oxygen scavenger is also needed to remove oxygen. There exist several methods to purify the raw material, depending on which process approach (solid treatment, liquid treatment, subliming, or all combination) and which scavengers (PbF2, CF4, and ZnF2) are used. The most popular process is called pre-melting, and this process is used prior to the actual melting/cooling process used to form the optical crystal. This pre-melt process consists of mixing the CaF2 powder with some amount of scavenger in powder form, heating to a temperature above the melting point, holding at the peak temperature (liquid state) for some time to allow the scavenger to react with oxygen and other impurities and then cooling down to solidify the melt as one ingot. In addition to the purification function, the pre-melting also serves to increase capacity because it provides a more dense starting material for the growth. As a simple, effective pre-treatment process, the pre-melting has been practiced in metal fluoride business for many years. In spite of this fact, there still exist some opportunities to improve the quality of the pre-melt. One of these areas is the scavenger retention problem in the pre-melt.
It is, within the context set forth above, with reference to the optimisation of the intervention of fluorinating agents, that the present invention has been developed.